The present invention refers to a method for transferring/receiving audio and/or video signals fulfilling the requirement to bridge synchronous and asynchronous networks and minimizing delay time over such networks.
It is known that during the last two decades audio transmission has been dominated by the high market penetration of dedicated and dial-up networks, such as E1, ISDN and fixed rate digital networks. In particular for live, but also for contribution and distribution, audio codecs (including encoder and decoder as well as network interfaces) are used via dial-up and dedicated lines for the purpose of encoding to reduce the bit rate and to adapt the digital signal to the network needs. Typically the applied bit rates are 56/64 kbps up to 2 Mbit/s.
In connection with the continuing process of integration and digitalization, in particular the high investments in LAN/WAN environments, a concept of audio gateway codecs with new possibilities is introduced: flexible use of data capacity by changing parameters like quality, bit rate and delay time of the digital audio signal. In ATM networks, high capacities can be used to transmit linear high quality audio without nearly any delay time. During high network traffic and due to more programs audio, may be coded with MPEG or other algorithms.
Furthermore high attention is given to an important issue for the broadcasting industry: compatibility between transmission and reception devices.
While at the end of 1992 MPEG 1 has been already standardized, other previously developed algorithms for compressed audio have been used in the eighties—first of all with an emphasis on speech.
Beside many advantages using the MPEG-algorithms, such as bit rate reduction, possibility to vary parameters such as bit rate, quality, delay time, there are disadvantages such as reduced number of encodes/decodes, so called cascadings, introduction of necessary n-1-mixing-technique for the area of monitoring in order to provide the possibility for reporters to hear themselves via headphone on air as well as the dramatically reduced possibilities to control quality, because typically there is no information about the already used bit rates of previous transmissions/storages, so called generations.
Up to the mid of the nineties, MPEG has been introduced in the area of audio transmission/reception, processing and storage. Later, a focus on certain applications has resulted in the use of more and more linear audio coding for recording/archiving, production.
An overview of applications and coding algorithm is given in Table 1 with additional information of the mainly used audio coding scheme. In addition to MPEG, other coding algorithms have to be considered.
TABLE 1ApplicationApplied coding schemeArchivingLinear/apt-XBroadcastMPEG/LinearautomationInterimMPEG/LinearstorageAudio-On-MPEGDemandReportingMPEGContributionMPEG/J.41/Linear/apt-XDistributionMPEG/J.41/Linear/apt-X
The audio scheme highly demanded is the linear audio coding for communication and storage due to extremely low delay time and high quality. Additionally, linear audio coding allows cascading. Alternatively, there can be used other algorithms, which are based on reduction of redundancy and also provide a low coding delay time, e.g. apt-X. In particular for applications in communications, low bit rate coding schemes are the preferred ones, because this is the only way to transmit signals in a cost effective way.
Applications of IP (Internet Protocol)-Audio have not been introduced in professional broadcasting, in particular due to the missing “Quality of Service QoS”.
During the last years, two segments of broadcasting and studio applications have been established nearly independently, although a combination might have been more useful: Communications and Production. The FIG. 1 shows the relation between three considered worlds in broadcasting:
Information Technology World (IT Production, LAN/WAN, Internet, Intranet);
Communications World (e.g. ISDN);
Broadcasting World (switching).
When transmitting files, there is no need for real-time, although there are less and more important files to be transmitted. Such files can be transmitted slower or faster than real-time. Until now file transmission is not used for Live-transmissions or On Air events.
As regards the link between Broadcasting-World and Communications World, the real-time transmissions are the most important ones. Input and output signals are routed in a switching room and—depending on bandwidth and destination—are sent via different networks. Many lines, e.g. for distribution to transmitters are used 24 hours 7 days—meaning all the time. Others, e.g. reporting only for a few minutes, e.g. via ISDN. Beside the fact that Live signals have to be considered, the overall coding delay obtains high attention.
If audio signals from LAN/WAN-networks of IT World need to be transmitted in real-time (live), typically the Broadcasting World is chosen and practically is used the chain shown in FIG. 2.
For the popular case, that a reporting shall be recorded, is chosen the chain shown in FIG. 3.
While in the first case, probably a already encoded file needs to be decoded first of all, in order to be transmitted via linar digital or analog and then needs to be input into another encoder for encoding purposes, the second example might even result in a signal being received via ISDN as MPEG signal, being decoded by the decoder to a linear signal and than being stored, e.g. as DAT and later being brought into the LAN and then probably again will be encoded into an MPEG file and stored on a server.
Both examples show cascading, integration of complex technique and thus high costs and investments.
A classification and consideration of the impact of delay time for various applications has been made. Table 2 shows applications which have requirements to delay time. There are advantages and disadvantages to use MPEG-algorithms, as well as algorithms, such as apt-X.
TABLE 2Pro-/Con withAltern.ApplicationRequirementMPEGalgorithmMonitoring <20 ms+cost forLinear, J.41,n-1-mixJ.57, apt-XInterview <80 ms+costLinear, J.41,−Delay highJ.57, apt-XG.722 qualitywith 7 k too lowReporting <80 ms+costLinear, J.41,+Asymm. CodecJ.57, apt-XG.722 qualitywith 7 k too lowDistribution>>100 ms+costLinear, J.57,+qualityEnhanced-apt-X−cascadingContribution>>100 ms+costLinear, J.57,+qualityEnhanced-apt-X−cascadingEmission>>100 ms+bandwidth+quality
There are principally differences: while MPEG includes system delay always, apt-X includes only very short delay time. There are also differences within the MPEG-standards, e.g. between Layer 2 and 3, between MPEG 1 and 2.
Today there are no professional audio codec devices fulfilling the requirement to bridge synchronous and asynchronous networks and the requirements of low delay.